Wireless charging technology can wirelessly transmit power between electronic devices and is therefore widely used in consumer electronics and other types of electronic products. Wireless charging technology usually achieves wireless transmission of the power through mutual electromagnetic coupling between a coil in a wireless power transmitting terminal and a coil in a wireless power receiving terminal.
The wireless power transmitting terminal converts a direct current voltage into an alternating current that generates an alternating magnetic field through the coil in the wireless power transmitting terminal. The wireless power receiving terminal couples to the alternating magnetic field to induce a corresponding alternating voltage that is then converted into a direct current voltage by a rectifying circuit to charge an electronic device. The induced voltage generated by the wireless power receiving terminal coupling to the alternating magnetic field is:Us=ωMIp(wherein=k√{square root over (LpLs)})
where ω is the frequency of the alternating magnetic field, M is a coupling inductance between a power transmitting coil and a power receiving coil, Ip is the current in the power transmitting coil which characterizes the intensity of the magnetic field, Lp is an inductance value of the power transmitting coil, Ls is an inductance value of the power receiving coil and k is a coupling coefficient between the power transmitting coil and the power receiving coil.
It can be known from the above equation that the induced voltage at the wireless power receiving terminal can be increased by increasing the coupling inductance M. The coupling inductance M can be increased by increasing the inductance value of the power transmitting coil, the inductance value of the power receiving coil or the coupling coefficient between the power transmitting coil and the power receiving coil. The inductance values and coupling coefficient of the coils can be increased by increasing the number of turns of the coils (the inductance value of the coil is directly proportional to the square of the number of turns). As shown in FIG. 1, the number of turns of the coil is increased from 4 to 8 in the same plane. In this way, the number of turns is increased by 2 times and the inductance value is increased by nearly 4 times. However, increasing the number of turns of the coil in the same plane will increase the area of the coil. If the area of the power transmitting coil and the area of the power receiving coil do not correspond to each other after increasing the number of turns of the coils, it will result in an increase in relative proportion of the area of the power transmitting coil and the area of the power receiving coil, and thus resulting in a decrease in the coupling coefficient k and thus a lower charging efficiency.